The present invention relates to the binders used in conventional powder metallurgy processes and in the three-dimensional fabrication processes known as xe2x80x9cSolid Free Form Fabricationxe2x80x9d processes.
The manufacture of metal dies and parts or molds for plastic injection molding may be produced by one of several conventional processes, including a process in which steel is machined into a desired mold shape and size by machine tool, by precision casting, or by special processing in which the mold material is treated by chemical etching or electrical spark discharge to attain a desired shape. These processes require complex manufacturing steps, highly skilled personnel and a great deal of time and, accordingly, are very expensive. Rapid manufacturing is a state-of-the-art method for producing parts and tooling quickly and automatically using a machine, which directly uses data from 3D CAD models or other sources of 3D geometry information to build a complete article in a layer-by-layer manner. There are two patented examples of rapid manufacturing or prototyping technologies which use powder materials to build articles. The three-dimensional printing process is described in U.S. Pat. No. 5,807,437 and the selective laser sintering process is described in U.S. Pat. No. 4,863,538, each of which is incorporated herein by reference.
Solid free form fabrication allows the creation of solid articles having interior channels and other structures or voids that would be impossible to cast by conventional molding or casting techniques. Many different materials may be used in the ultimate fabrication, including ceramics, metals and metal/ceramics, but the most useful typically are the steel and other metal or composite alloys which can be created by (1) binding metal or metal/ceramic powders in the desired three-dimensional construct; (2) sintering the bound particles; and (3) infiltrating into the sintered construct additional molten metals such as molten copper, molten bronze, etc.
The present invention is directed to a choice of binder to enhance the overall performance of previously known solid free form fabrication methods of all types. These methods include, but are not limited to, selective laser sintering, laminated object manufacturing, fused deposition modeling, precision optical manufacturing, directed light manufacturing, light engineered net shaping, three-dimensional printing, and others. Many of these processes have already been disclosed in, for example, the above-cited and other United States Patents. Typically, three-dimensional printing proceeds similarly to ink-jet printing, but instead of ink a binder is printed onto a powder layer following a computer pattern that is obtained by applying a slicing algorithm to the computer model of the article.
A challenge in most or all of these prior art processes has been the handling of the construct after binding but prior to sintering. Bound constructs not yet sintered are generally referred to as xe2x80x9cgreen,xe2x80x9d in accordance with historic usage in the ceramics industry. For example, when metal or metal/ceramic composite powders are deposited layer-by-layer in a three-dimensional printing process, an acrylic polymer solution or emulsion is typically used as the binder layer as multiple alternating layers of powder and binder are deposited. Acrylic polymers, however, as is typical of many polymers, disintegrate at about 500 degrees C. The use of acrylic polymer binder thus dictates that the binding step itself can be conducted at a temperature no higher than about 500 degrees C. Typically, therefore, the green constructs bound with acrylic binder are fragile enough to warrant exceedingly careful handling, because at temperatures lower than about 500 degrees C. typically no particle fusion has begun at all and the construct is held together only by the cured binder. Breakage or damage of green constructs must be carefully avoided due to their relative fragility. Also, it is believed that the heat-deteriorated polymer binderxe2x80x94if anything is left of it at all after sinteringxe2x80x94does not contribute ultimately to the strength or quality of the final sintered and infiltrated product. An ideal binder composition would not only improve the green strength of the construct but also its ultimate strength and other alloy characteristics. An additional consideration in the selection of binders for solid free form fabrication processes is whether the binder composition is shelf stable and safe, that is, whether it poses a materials safety problem or generates any hazardous waste.
In order to meet this need, the present invention is a binder, and a method of using it in solid free form fabrication including metal powder, in which the binder contains at least one carbohydrate as the active binding compound. The carbohydrate generally contains between 6 and about 900 carbon atoms and may be selected from various categories including but not limited to: 1) monosaccharides such as the aldose and ketose sugars and their glycoside, hydroxyl, methyl, acyl, carbonyl, phosphate, deoxy, amino and other derivatives; 2) disaccharides such as sucrose, maltose, lactose, dextrose, cellobiose, gentiobiose, and trehalose; 3) trisaccharides such as raffinose and melezitose; 4) polysaccharides containing the base sugars identified in 1)-3) above; and 5) hydrolyzed starches in which the hydrolysate contains between about 6-900 carbon atoms, including dextrins such as limit dextrin, hydrolyzed amylose, and hydrolyzed amylopectin. Carbohydrates which form isomers may be in either D- or L-form. In general, the carbohydrate is usually dissolved or dispersed (i.e., hydrated micelles) in an aqueous carrier solution but any carrier may be used as long as the carbohydrate can be distributed therein.